1. Field of Invention
This invention relates to photovoltaic solar cell arrays, specifically to a shadowing timer that controls the output power of a solar cell array on two independent time scales: time of day and time of year.
2. Description of Prior Art
Small photovoltaic solar cell arrays (one to one thousand Watts peak power) are an economical and reliable method of powering modest off-grid loads. These arrays are often connected directly to a load when used in ventilation, pumping, irrigation, and water circulation applications. Because of the direct electrical connection, these systems usually run during the entire time the solar cell array is exposed to direct sunlight. In some cases this long running time may not be necessary or desirable. A 3000-hour DC pump running 10 hours a day, for example, will exceed its rated lifetime in under a year. Over periods ranging from years to decades, the cost of maintenance and component replacement can far exceed the cost of the solar cell array.
Furthermore, many applications only require energy during a certain time of year or certain time of day. For example, an attic ventilation fan should run only during the summer months, while a defrosting or de-icing device only requires energy during the winter. It may only be necessary to run an irrigation pump for an hour each sunny day, and it may be preferable to shut the pump off completely during the winter to prevent ice damage.
Previous methods of limiting component operation have relied on electrical switching devices such as timers, float switches, thermostats, and user-operated switches. Timers drift and usually require a continuous power supply. Float switches, thermostats, and other feedback controls increase the cost and complexity of a system and are prone to failure. Malfunctioning feedback controls can damage system components and endanger other property.
U.S. Pat. No. 4,453,534 to Kazimir (1984) for a solar water heating system illustrates the use of both a timer and a photovoltaic solar cell array as alternative methods for controlling a circulation pump. Kazimir suggests using "a timer set to turn said pump on at a predetermined time of day and to turn said pump off at a second predetermined time of day." When Kazimir substitutes a solar cell array in place of the timer, he does not attempt to optimize the system; the pump simply runs whenever the array is exposed to sufficient sunlight.
The prior use of shading devices is also relevant to the shadowing timer. Shading devices, such as vanes, boxes, and tubes, are widely used to shadow light sensors and thereby increase the sensor sensitivity to the angle of incident light. U.S. Pat. No. 4,107,521 to Winders (1978) and U.S. Pat. No. 4,151,408 to Brown et al. (1979) incorporate sensor shades to improve the accuracy of solar tracking devices. The solar tracking apparatus in U.S. Pat. No. 4,556,788 to Hanak (1983) uses shadow vanes to shade one half of a solar cell array, thereby generating a differential voltage that drives a solar tracking apparatus. Hanak's apparatus does not function as a timer and does not derive any benefit from the changing direction of incident light as the sun moves through diurnal and annual cycles.